EFFECTS OF SEX-RATIO ON INTRA-SEXUAL AND INTER-SEXUAL BEHAVIOR IN SAND GOBIES

Reproductive behaviour and reproductive success in a marine fish, the sand goby, Pomatoschistus minutus, were affected by changes in the sex ratio. When the sex ratio was male biased (six males:three females), aggressive male-male interactions were more frequent per individual th an in a female-biased situation (three males:six females). Accordingly , in the female-biased treatment females interacted more with each oth er than in the male-biased treatment. There was no difference between treatments in male interactions towards females, nor in female interac tions towards males. Controls, with even sex ratios at two different d ensities (three or six of each sex), did not differ from each other in intra- or inter-sexual interactions. This shows that the differences in intra-sexual behaviour, found in males as well as females, were cau sed by the sex ratio and not by density. In the male-biased treatment and in the unbiased controls, nest-building males were larger than non -building males, whereas in the female-biased treatment there was no d ifference in size between builders and non-builders. Thus, our experim ents show that a change in sex ratio changes competition for mates, an d that individuals adjust their reproductive behaviour to the current sex ratio. (C) 1995 The Association for the Study of Animal Behaviour